Bycatch and discards
Environment · Last reviewed 2026-08-10
Everything a fishery catches that it did not intend to — and the area where gear design has produced some of the clearest environmental wins available.
Bycatch is the catch a fishery takes that it was not targeting: undersized fish of the target species, non-target fish, and — most consequentially — seabirds, turtles, sharks and marine mammals. Discards are the portion of the catch returned to the sea, usually dead or dying.
The scale and character vary enormously by gear, and this is the point on which public discussion is most often incoherent. A pot fishery has very low bycatch and returns most of it alive. A purse seine set on a free-swimming tuna school has low bycatch; the same gear set around a floating aggregating device has substantially more. A pelagic longline is highly selective for its target size and has historically been a major cause of seabird mortality. Bottom trawling's bycatch profile depends on mesh, on selectivity devices, and on the ground being worked.
What makes bycatch an unusually hopeful topic is that it responds to engineering. Bird-scaring streamer lines, weighted longlines and night setting have produced very large reductions in seabird mortality in fisheries that adopted them. Square-mesh panels, sorting grids and escape gaps let undersized fish and non-target species out of nets and pots. Circle hooks reduce deep hooking and improve the survival of what is released.
Recreational angling has its own version, and it is worth naming: undersized and non-target fish caught and returned, whose survival depends on the handling described elsewhere in this catalogue.
The seabird case, and why it is worth knowing
Longline fisheries hook seabirds during setting, when baited hooks are at the surface and birds dive for them. For albatrosses and petrels — long-lived, slow-breeding species where adult mortality matters disproportionately — this was a significant driver of population decline.
The response was technical rather than prohibitive. Streamer lines flown behind the vessel keep birds away from the sinking hooks; weighting the line makes the hooks sink faster and out of reach; and setting at night removes most of the birds' ability to see the bait. Where these measures are properly applied, seabird bycatch falls dramatically.
It is worth knowing for two reasons. It is an example of an environmental problem in fisheries being substantially solved by changing the gear rather than by stopping the activity — which is not always available, and is worth recognising when it is. And it demonstrates the general principle that runs through this whole topic: the gear, and how it is used, matters far more than the category of fishery it belongs to.
Authoritative guidance
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: The mitigation measures used to reduce seabird bycatch in longline fisheries, and the mechanism by which they work.States this directlyCitation checked — the work exists and is described correctly
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: That bycatch is addressed through technical measures on gear alongside catch limits in managed fisheries.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
Where to go next
Species affected
- Blue sharkThe blue shark is the most heavily caught shark in world fisheries, overwhelmingly as pelagic longline bycatch rather than as a target, which is the context every recreational conversation about it sits inside.
- Spotted eagle rayTaken as bycatch across much of its range in coastal fisheries, which for a species replacing perhaps two individuals per female per year is not a marginal pressure.
- TopeA highly migratory shark taken across its range as bycatch, which is why national measures alone cannot rebuild the stock a British angler catches from.
- Yellowfin tunaThe species' association with porpoises and floating objects is exactly what made purse seining for it so efficient and so damaging, and the resulting bycatch problem reshaped an entire industry.
What these connections rest on (4)
Scientific evidence — 3 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Blue shark
An environmental effect attached to the thing that causes it, resting on an authoritative source rather than on advocacy.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: The global Red List assessment for the species concerned, and the population trend and threats recorded in it.States this directlyCitation checked — the work exists and is described correctly
- JNCC species and habitat conservation advice — Joint Nature Conservation Committee (Conservation authority)Relied on for: UK conservation designation and the status of protected and priority species.States this directlyCitation checked — the work exists and is described correctly
Spotted eagle ray
A conservation claim about a species with a documented assessment, joined to the environmental issue it exemplifies.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: The global Red List category and the assessment behind it, including the reproductive and habitat factors driving it.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible capture, including selectivity, the treatment of berried and undersized animals, and the precautionary approach.Background and mechanism onlyCitation checked — the work exists and is described correctly
Tope
A conservation claim about a species with a documented assessment, joined to the environmental issue it exemplifies.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: The global Red List category and the population assessment behind it for the species concerned.States this directlyCitation checked — the work exists and is described correctly
- Cefas science, advice and data on marine and freshwater environments — Centre for Environment, Fisheries and Aquaculture Science (Government fisheries body)Relied on for: UK marine fisheries science: survey data, distribution and elasmobranch stock monitoring in UK waters.States this directlyCitation checked — the work exists and is described correctly
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Yellowfin tuna
An environmental claim resting on the documented association plus the gear literature, held to an authoritative standard.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: That larger Thunnus albacares frequently school with porpoises and that the species associates with floating debris and other objects.States this directlyPassage read and verified
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: The classification of purse seine gear and the bycatch characteristics associated with encircling gear.States this directlyCitation checked — the work exists and is described correctly
Practices involved
- LongliningThe seabird case is the clearest example available of a fisheries bycatch problem substantially solved by changing the gear.
- Purse seiningBycatch here depends on how the gear is SET rather than on the gear itself, which is the point most often lost in public discussion.
- Traditional net fishingSelectivity in netting is almost entirely a function of mesh size and of where and when the net is worked.
- TrawlingTowing a large net through the water or along the seabed — the dominant commercial fishing method by volume, and the most environmentally contested.
What these connections rest on (4)
Authoritative guidance — 4 connections
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Longlining
Seabird and shark bycatch in longline fisheries, and the mitigation measures addressing them, are documented.
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Longline bycatch and the mitigation measures — streamer lines, weighting, night setting — used to reduce it.States this directlyCitation checked — the work exists and is described correctly
Purse seining
Free-school and aggregating-device sets differ substantially in bycatch composition.
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: The distinction between free-school and aggregating-device purse seine sets in bycatch terms.States this directlyCitation checked — the work exists and is described correctly
- Good Fish Guide — Marine Conservation Society (Conservation authority)Relied on for: That sustainability ratings differ by set type as well as by stock.States this directlyCitation checked — the work exists and is described correctly
Traditional net fishing
Net selectivity characteristics are documented by gear type.
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: The selectivity of net gear and its dependence on mesh and deployment.States this directlyCitation checked — the work exists and is described correctly
Trawling
Bycatch and discarding are recognised issues in trawl fisheries and are addressed through gear selectivity measures.
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Trawl gear mechanics, selectivity devices and their effect on unwanted catch.States this directlyCitation checked — the work exists and is described correctly
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: That bycatch is managed through technical measures alongside catch limits.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
Where this applies
- The Finnish lake districtThe seal gear restrictions are a bycatch problem in its purest form: the gear is legal, the target is legal, and the harm falls on a non-target animal small enough in number that individual mortalities matter to the population.
- The Humboldt CurrentThe 1970s collapse of the anchoveta fishery, under heavy fishing coinciding with a strong El Niño, is one of the foundational cases in fisheries science.
- The Indian subcontinent coastsTrawl and gillnet fisheries on this coast historically landed a very large share of the world's shark and ray catch, much of it as bycatch.
- The MaldivesPole-and-line tuna fishing takes one fish at a time from a visible surface school and produces almost no bycatch, which is why it is repeatedly cited as the low-impact alternative to purse seining.
- The Mekong systemFence and trap gear set across draining floodplain channels is inherently unselective, taking whatever the falling water delivers.
- The Severn EstuaryFixed engines set to take fish on the ebb are inherently unselective, and their progressive reduction as salmon stocks declined is an example of ancient rights constrained by modern stock assessment.
- The Strait of MessinaLarge-scale driftnetting for swordfish here took enormous quantities of cetaceans, turtles and sharks and was prohibited, and the contrast with harpooning is one of the sharpest illustrations of gear selectivity available.
- The West African coastIndustrial trawling incursions into the inshore band take juveniles from nursery ground and destroy artisanal gear, which is a documented and recurring conflict.
What these connections rest on (8)
Scientific evidence — 8 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
The Finnish lake district
A claim that a place raises a particular environmental question, resting on the scientific and conservation literature for the system rather than on angling practice.
- JNCC species and habitat conservation advice — Joint Nature Conservation Committee (Conservation authority)Relied on for: Habitat classification and description for the intertidal, coastal and freshwater habitats named, and the species characteristically associated with them.States this directlyCitation checked — the work exists and is described correctly
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Ecoregion description, nursery function and the hydrography of the European sea areas assessed.States this directlyCitation checked — the work exists and is described correctly
The Humboldt Current
An environmental pressure that is specific to this place rather than general to fishing, joined so the issue record carries a real example.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Assessment of the species and habitats affected, and the pressures acting on them, including habitat change and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible fisheries management, precaution, gear selectivity and the treatment of shared and small-scale fisheries.Background and mechanism onlyCitation checked — the work exists and is described correctly
The Indian subcontinent coasts
An environmental pressure that is specific to this place rather than general to fishing, joined so the issue record carries a real example.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Assessment of the species and habitats affected, and the pressures acting on them, including habitat change and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible fisheries management, precaution, gear selectivity and the treatment of shared and small-scale fisheries.Background and mechanism onlyCitation checked — the work exists and is described correctly
The Maldives
An environmental pressure that is specific to this place rather than general to fishing, joined so the issue record carries a real example.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Assessment of the species and habitats affected, and the pressures acting on them, including habitat change and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible fisheries management, precaution, gear selectivity and the treatment of shared and small-scale fisheries.Background and mechanism onlyCitation checked — the work exists and is described correctly
The Mekong system
An environmental pressure that is specific to this place rather than general to fishing, joined so the issue record carries a real example.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Assessment of the species and habitats affected, and the pressures acting on them, including habitat change and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible fisheries management, precaution, gear selectivity and the treatment of shared and small-scale fisheries.Background and mechanism onlyCitation checked — the work exists and is described correctly
The Severn Estuary
An environmental pressure that is specific to this place rather than general to fishing, joined so the issue record carries a real example.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Assessment of the species and habitats affected, and the pressures acting on them, including habitat change and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible fisheries management, precaution, gear selectivity and the treatment of shared and small-scale fisheries.Background and mechanism onlyCitation checked — the work exists and is described correctly
The Strait of Messina
An environmental pressure that is specific to this place rather than general to fishing, joined so the issue record carries a real example.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Assessment of the species and habitats affected, and the pressures acting on them, including habitat change and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible fisheries management, precaution, gear selectivity and the treatment of shared and small-scale fisheries.Background and mechanism onlyCitation checked — the work exists and is described correctly
The West African coast
An environmental pressure that is specific to this place rather than general to fishing, joined so the issue record carries a real example.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Assessment of the species and habitats affected, and the pressures acting on them, including habitat change and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: Principles for responsible fisheries management, precaution, gear selectivity and the treatment of shared and small-scale fisheries.Background and mechanism onlyCitation checked — the work exists and is described correctly
How this compares
What these connections rest on (1)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Selectivity and what it is used for
A gear-selectivity comparison held to an authoritative standard because it underpins most of the public argument about fishing methods.
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: The classification of spear and harpoon gear and its selectivity characteristics relative to net and hook gear.States this directlyCitation checked — the work exists and is described correctly
- Good Fish Guide — Marine Conservation Society (Conservation authority)Relied on for: That gear type is a principal determinant of a fishery's sustainability rating.States this directlyCitation checked — the work exists and is described correctly
Sources for this page
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Gear-specific bycatch characteristics and the mitigation measures available for each.States this directlyCitation checked — the work exists and is described correctly
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: That bycatch is managed at stock and fishery level through technical measures.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
How FishingHQ labels evidence is explained in the evidence policy.